<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>闭包陷阱</title>
  </head>
  <body>
    <button id="btn">add</button>
    <script>
      function foo() {
        var el = document.getElementById("btn");
        el.onclick = function () {
          console.log(el.id);
        };
      }
      foo();
    </script>
  </body>
</html>
